This application claims a benefit of priority based on Japanese Patent Application No. 2003-048365, filed on Feb. 26, 2003, which is hereby incorporated by reference herein in its entirety as if fully set forth herein.
The present invention relates generally to precision machines for mounting a lens, and more particularly to a projection optical system in an exposure apparatus, etc. More specifically, the present invention relates to a retainer that holds an optical element for a more precise imaging relationship in an exposure apparatus in projecting an image on an original sheet, such as a mask or reticle onto an object, such as a single crystal substrate for a semiconductor wafer, a glass plate for a liquid crystal display (xe2x80x9cLCDxe2x80x9d). The exposure apparatus is used to fabricate a semiconductor device, an image pick-up device (such as a CCD), and a thin film magnetic head.
The fabrication of a device using the lithography technique has employed a projection exposure apparatus that uses a projection optical system to project a circuit pattern formed on a mask onto a wafer and the like, thereby transferring the circuit pattern. The projection optical system enables diffracted beams from the circuit pattern to interfere on the wafer and the like, so as to form an image.
The devices to be mounted on electronic apparatuses should be highly integrated to meet recent demands for miniaturization and low profile of electronic apparatuses, and finer circuit patterns to be transferred or higher resolution have been demanded increasingly. A short wavelength of a light source and an increased numerical aperture (xe2x80x9cNAxe2x80x9d) in a projection optical system are effective to the high resolution as well as a reduced aberration in the projection optical system.
An optical element, such as a lens and a mirror, when deforming in an projection optical system causes aberration because an optical path refracts before and after the deformation and light that is supposed to form an image at one point does not converge on one point. The aberration causes a positional offset and short-circuits a circuit pattern on a wafer. On the other hand, a wider pattern size to prevent short-circuiting is contradictory to a fine process. Therefore, a projection optical system with small aberration should hold its optical element(s) without changing a shape and a position relative to the optical axis of the optical element in the projection optical system so as to maximize the original optical performance of the optical element.
FIG. 12 is a schematic sectional view of a conventional retainer 1000 for holding an optical element 1100. Referring to FIG. 12, the optical element 1100 is engaged with or slightly spaced from an inner circumference of a retaining member 1200 so that one surface of the optical element 1100 contacts a support part 1200a provided on the retaining member 1200. Adhesive 1300 is inserted into an aperture between an outer circumference of the optical element 1100 and the inner circumference of the retaining member 1200. After the adhesive 1300 cures, the optical element 1100 held integrally by the retaining member 1200. The optical element 1100 thus held by the retainer 1000 can constitute an optical system held by a housing 2000 with other optical elements 1100 similarly held by the retainer 1000. FIG. 13 is a schematic sectional view of an optical system including the optical elements 1100 held by the conventional retainer 1000.
In general, the support part 1200a in the retaining member 1200, which contacts the optical element 1100 in a range of 360xc2x0 around a rotational center axis of the optical element 1100, as shown in FIG. 14. However, the mechanical processing has a difficulty in maintaining the support part 1200a to be completely flat, and the optical element 1100 contacts the retaining member 1200 at plural points from the microscopic viewpoint irrespective of a design of contact in a range of 360xc2x0. Subject to the gravity influence in this state, the optical element 1100 undulates by its own weight around contact points as vertices. In particular, a projection lens tends to have a larger caliber and a larger lens capacity due to the recent high NA in the projection optical system, and easily deforms by its own weight. Here, FIG. 14 is a schematic structure of the retaining member 1200 of the conventional retainer 1000.
The projection optical system corrects aberrations that result from various errors in plural optical elements, such as a mirror and a lens, by adjusting a combination of these optical elements and a positional relationship among them, and should consider nanometer deformations about a surface shape of the optical element. However, use of the retainer 1000 would change contact points between the used optical element 1100 and the retaining member 1200 according to a combination between them, and thus vary deformed surface shapes. Therefore, aberrations scatter among retainers that hold different optical elements, and corrections of the aberrations become very arduous.
A retaining member 1500 can be used, as shown in FIG. 15, which arranges three support parts 1500a at 120xc2x0 intervals around the rotational center of the optical element 1100. The optical element 1100 always contacts and is supported by three support parts 1500a arranged at 120xc2x0 intervals, is subject to the gravity force, and undulates around contact points (or these support parts 1500a) as vertices that are arranged at 120xc2x0 intervals. Here, FIG. 15 is a schematic structure of the remaining member 1500 in another conventional retainer.
Since a plane can be geometrically defined by three points, three support parts 1500a always define the same plane irrespective of the processing precision of the retaining member 1500. Therefore, the optical element 1100 that contacts these support parts 1500a is supported under approximately the same condition even when the retaining member 1500 is replaced with another retaining member 1500.
The optical element that undulates around three projections usually generates a trigonometric component of wave front aberration, but this aberration is correctable when the optical element is combined with another optical element in an optical system. This feature reduces scattering aberration among retainers that hold different optical elements, and facilitates an aberrational correction more easily than that for the retaining member 1200 that is configured to contact the optical element in a range of 360xc2x0 around the rotational center axis of the optical element.
An optical element, such as a mirror and a lens, is often made of an optical glass material, such as quartz, due to excellent optical characteristics and manufacture convenience, whereas a retaining member for the optical element is made of a metallic material for strength and processability. In other words, the optical element and retaining member are made of different materials and have different coefficient of linear expansions. Thus, when the temperatures of the optical element and retaining member vary, for example, as the ambient temperature of the optical system varies and as the optical element heats up, the optical element and the retaining member have different expansion and contraction amounts due to different coefficients of linear expansion.
In the conventional retainers shown in FIGS. 12 to 15, an outer circumference of the optical element and an inner circumference of the retaining member connected to the optical element via the adhesive have different expansion and contraction amounts, and the optical element is subject to a tensile or compressive compulsory displacement in a radial direction and its top and bottom surfaces deform. The optical element consequently changes its optical performance, and the optical system that includes plural optical elements also changes the optical performance. In other words, an optical apparatus deteriorates the optical performance as the temperature varies.
In particular, a retainer that is configured to contact an optical element in a range of 360xc2x0 around the rotational center axis of the optical element, microscopically contacts the optical element at plural differently positioned points, as discussed, and scatters shape changes of the optical elements as the temperature varies. As a result, it is very difficult to predict aberrational changes and correct the aberration as the temperature varies.
On the other hand, the retainer that is configured to contact an optical element at three points changes a shape of the optical element because a difference in expansion and contraction between the optical element and the retaining member provides a compulsory displacement to the outer circumference of the optical element, but does not displace undulated vertices in the rotational direction so that a size of the projection increases and decreases. Thereby, the trigonometric component of the wave front aberration varies with the temperature due to deformations of the optical element, but it is difficult to correct a variance amount of the trigonometric component as the temperature varies.
Accordingly, it is an exemplified object of the present invention to provide a retainer that restrains changes of trigonometric components generated in an optical element when an ambient temperature changes, and prevents deteriorations of optical performance.
A retainer of another aspect according to the present invention for holding an optical element, the optical element according a center axis of the optical element with a gravity direction, and having an approximately rotationally symmetrical shape includes a retaining member that includes three support parts arranged at approximately 120xc2x0 intervals around the center axis, and holds the optical element via the support parts, and a joint member that joints the optical element with the retaining member, wherein |(zbxe2x88x920.6wb)xe2x88x92(zg+1.2)|xe2x89xa61 is met, where a Z coordinate system has an origin at an intersection between the center axis and a surface of the optical element, which surface faces a direction opposite to the gravity direction, and sets a Z axis to be positive in the direction opposite to the gravity direction of the center axis, zg is a coordinate of a gravity center of the optical element in the Z coordinate system, zb is a coordinate of a center position of a width of said joint member in the z axis direction by which said joint member contacts the optical element in the Z coordinate system, and wb is the width of said joint member in the z axis direction by which said joint member contacts the optical element.
The optical element is, for example, a mirror. The joint member may be an adhesive or a comb-shaped spring. The retaining member may have an annular shape around the center axis of the optical member. The joint member may joints the retaining member around an entire outer peripheral of the optical element.
An optical system of another aspect according to the present invention includes an optical element that accords a center axis with a gravity direction, and has an approximately rotationally symmetrical shape, and the above retainer. An optical apparatus of still another aspect according to the present invention may include plural optical elements, and the above retainer for holding at least one of said optical elements.
An exposure apparatus of another aspect according to the present invention includes the above retainer, and an optical system for exposing a pattern formed on a mask or reticle onto an object via the optical element held by the retainer.
A device fabrication method of another aspect of the present invention includes the steps of exposing a pattern on a mask, onto an object by using the above exposure apparatus, and developing the exposed object. Claims for the device fabrication method that exhibits operations similar to those of the above exposure apparatus cover devices as their intermediate products and finished products. Moreover, such devices include semiconductor chips such as LSIs and VLSIs, CCDs, LCDs, magnetic sensors, thin-film magnetic heads, etc.
Other objects and further features of the present invention will become readily apparent from the following description of the embodiments with reference to accompanying drawings.